The present invention relates to a steel sheet coating with a Zn--Mg binary coating layer excellent in corrosion resistance, anti-powdering, adhesiveness, spot weldability, anti-discoloring and water-proof secondary paint adhesiveness. The present invention is also concerned with a method of manufacturing said Zn--Mg alloy coated steel sheet by vacuum deposition process.
Various finishing methods have been developed so far in order to improve the corrosion resistance of a steel sheet. A representative method is Zn plating. Zn plating is performed by electroplating or hot-dip coating. A demand for corrosion resistant material having higher properties becomes stronger year by year, in response to the pollution of the atmosphere. In this regard, various improvements have been proposed in electroplating and hot-dip coating.
In order to improve the corrosion resistance of a Zn-coated steel sheet manufactured by hot-dip coating process, what is thought at first is the increase of the adhesion amount of a Zn layer. However, there is a limitation on the increase of the adhesion amount due to operational conditions, so that corrosion resistance is improved to a limited extent by the increase of the adhesion amount. On the other hand, the increase of the adhesion amount, i.e. making a plating layer thicker, is likely to cause defects such as scuffing and flaking during press-working the coated steel sheet.
In order to form a coating layer with high adhesion amount by electroplating process, a line speed is necessarily determined lower, or electrolytic cells are obligatorily increased in number. Consequently, productivity is significantly reduced.
The corrosion resistance can be improved by depositing a Zn alloy such as a Zn--Ni alloy. However, since the Zn--Ni alloy layer is hard and fragile, defects such as cracking or chipping are likely to be formed in the coating layer during working the coated steel. When these defects are formed in the coating layer, the substrate steel is exposed to the atmosphere through the defects. Consequently, the property of the coating layer itself is not exhibited well, and the defects acts as the starting points to develop corrosion.
A vapor deposition process is highlighted as the method to overcome the abovementioned problems in the hot-dip coating or electroplating process. Especially, a Zn--Mg alloy-coated steel sheet is expected as superior corrosion resistant material.
For instance, Japanese Patent Application Laid-Open 64-17853 discloses the formation of a Zn--Mg alloy coating layer containing 0.5-40 wt. % Mg and the coating layer mainly composed of Zn--Mg intermetallic compounds effective in affinity to paint. Japanese Patent Application Laid-Open 2-141588 discloses the improvement of the Zn--Mg alloy coating layer in adhesiveness and workability by the formation of an intermediate layer such as Zn, Ni. Cu, Mg, Al, Fe, Co or Ti between the coating layer and the substrate steel. Japanese Patent Application Laid-Open 64-25990 discloses the provision of a Zn--Ti alloy layer on a Zn--Mg alloy coating layer to improve corrosion resistance after painting.
A vapor deposition Zn-coated steel sheet is continuously manufactured by reductively heating a steel sheet in the same unoxidizing and reducing furnaces as those in a conventional hot-dip process and then vapor depositing one or more coating metals, as disclosed in Nisshin Tech. Review No. 56 (1987), p. 41. When the steel sheet is heated in the nonoxidizing furnace, oils remaining on the surface of the steel sheet are burnt and removed from the surface. The steel sheet is then annealed in the reducing furnace held in a gas atmosphere such as H.sub.2 --N.sub.2 or H.sub.2, so that oxide films are decomposed and separated from the surface of the steel sheet. The steel sheet having the surface activated in this way is cooled in a reducing atmosphere and carried through a duct held in a N.sub.2 atmosphere and vacuum sealing means into a vacuum chamber. In the vacuum chamber, Zn is vapor deposited on the steel sheet, and the steel sheet is carried out through outlet vacuum sealing means.
Since the aforementioned process for manufacturing vapor deposition Zn-coated steel sheets passes through the same pretreatment step as that in a conventional hot-dip coating process, a vapor deposition coating process can be performed using a part of existing equipment. The equipment including the vapor deposition step may be applied to the production of a Zn--Mg alloy coated steel sheet excellent in corrosion resistance. A degreasing-cleaning cell may be used instead of the unoxidizing furnace. The vapor deposition Zn coating method has the same or higher efficiency compared with a Zn electroplating process.
A steel sheet coated with a Zn--Mg alloy layer in big adhesion amount has the defect that powdering is likely to form during press working the coated sheet sheet. Said powdering is accelerated, when the rate of Zn--Mg intermetallic compounds in the coating layer is bigger as the increase of Mg concentration, or when there are intermetallic compounds near the boundary between the coating layer and substrate steel even if Mg concentration is lower. The powdering is caused by hard and fragile Zn--Mg intermetallic compounds which can not follow the deformation of the substrate steel having high ductility and forms interlayer splitting or cracking in the end.
The powdering may be eliminated by the decrease of Mg concentration for reducing intermetallic compounds in the coating layer and for enhancing the ductility of the coating layer. However, the decrease of Mg concentration deteriorates the corrosion inhibiting power of the coating layer. Although the powdering can be suppressed by increasing Mg concentration only at the top layer, the Mg-enriched surface is colored black resulting in the reduction of commercial value. Besides, the high Mg concentration at the surface of the coating layer accelerates the diffusion of Mg to a welding electrode during spot welding, so that the coated steel sheet shows poor weldability.
When a Zn--Mg alloy coated steel sheet is manufactured by depositing Mg and then Zn, and heating the deposition layer to promote mutual diffusion between Mg and Zn, the steel sheet is heated in the reducing atmosphere to remove oxide films from the surface. However, when the steel sheet having the surface activated is carried through a duct held in a N.sub.2 atmosphere, the surface is contaminated and re-oxidized by O.sub.2 and H.sub.2 O slightly remaining in the N.sub.2 atmosphere. The formed oxide films react with Mg, Fe and Zn and form brittle reaction products. Consequently, the coating layer formed on the substrate steel sometimes shows poor adhesiveness.
When vapor deposition is performed on a steel sheet held at a relatively lower temperature, vacancies are likely to be formed in the coating layer. The resultant coating layer is not dense, and the substrate steel is exposed to a corrosive atmosphere through the porous coating layer. Consequently, the coating layer does not exhibit its corrosion inhibiting effect well.
The Zn--Mg alloy coating layer, different from a coating layer formed by conventional hot-dip coating or electroplating process, remarkably changes its properties in response to its lamellar structure. A steel sheet coated with a Zn--Mg binary coating layer having the laminated structure that a high-Mg sublayer formed at the middle is sandwiched with low-Mg sublayers sometimes shows poor water-proof secondary paint adhesiveness. For instance, the effective adhesiveness of paint film is not obtained as the result of the test that the coated steel sheet after being painted is dipped in warm water of 50.degree. C. for a long time. Merely the corrosion resistance can be improved by locating the high-Mg sublayer at the top without the formation of the low-Mg sublayer at the top. However, the high-Mg sublayer existing at the top of the coating layer deteriorates the water-proof secondary paint adhesiveness, but also promotes discoloration due to dump.